Hybrid vehicles have an electric drive (EV) mode, in which the hybrid vehicle travels only by power from the motor-generator, and a hybrid drive (HEV) mode, in which the hybrid vehicle can travel by power from both the engine and the motor-generator. Hitherto, various types of hybrid driving devices used in hybrid vehicles like the one described above have been proposed.
One type such as that described in Japanese Unexamined Patent Application Publication No. 11-082260 among the various types is known. The hybrid driving device disclosed in that document includes a motor-generator provided between an engine and a transmission as a result of connecting an output shaft of the engine and an input shaft of the transmission. A first clutch that connects the engine and the motor-generator so that they can be separated from each other, and a second clutch that connects an output shaft of the transmission and the motor/generator so that they can be separated from each other.
When the first clutch is disengaged, and the second clutch is engaged, the hybrid vehicle including the hybrid driving device is put in the electric drive (EV) mode, in which the hybrid vehicle travels only by power from the motor-generator. When the first and second clutches are both engaged, the hybrid vehicle including the hybrid driving device is put in the hybrid drive (HEV) mode, in which the hybrid vehicle can travel by power from both the engine and the motor-generator.
In the hybrid vehicle, when engine output becomes unnecessary because it has become possible to generate a required driving force only by the motor-generator as a result of a reduction in the required driving force. Such a reduction is caused by, for example, a driver taking is his/her foot off a depressed accelerator pedal when the hybrid vehicle is traveling in the HEV mode. Then, the HEV mode is switched to the EV mode.
According to this known device, it is necessary to perform the mode switching while disengaging the first clutch and stopping the engine. In addition, it is necessary to perform driving force proportion switching so that a state in which the required driving force is generated by the engine and the motor-generator is switched to a state in which the required driving force is generated by only the motor-generator.